1. Field of the Invention
The present invention relates to an apparatus for driving a light emitting device, which is applicable to an illumination device or a backlight unit (BLU), and more particularly, to an apparatus for driving a light emitting device using a pulse-width modulation (PWM), in which a driving control transistor provided in each channel in order to control the driving of a multi-channel light emitting device operates in a PWM scheme.
2. Description of the Related Art
Light emitting devices (LEDs) have been applied in various fields, for example, illumination devices or backlight units, and their applications are currently being expanded.
In such LED backlight units, a multi-channel LED driving scheme is used for a local dimming function and a scanning function. Also, a linear scheme is used for maintaining a constant level of brightness.
The linear scheme is advantageous in terms of price, but is problematic in terms of heat generation in a driver IC due to an LED forward voltage (VF) deviation between channels. Thus, there is a limitation in embedding a multi-channel LED driver circuit into an IC.
A conventional multi-channel LED driver circuit has a plurality of channels there inside in order to drive a plurality of LEDs, and senses a current flowing through each channel and controls a current in a linear scheme.
Meanwhile, due to the LED forward voltages deviation, different voltages are applied to LED strings of the multi-channels. An operating voltage (Vcc) is controlled by feeding back the lowest LED string voltage.
However, in the conventional multi-channel LED driver circuit, the forward voltage deviation exists between the LED strings, and a high voltage is applied to LED driving control elements (transistors) by the forward voltage deviation. Thus, a great deal of heat is generated in the driving control elements.
Due to the heat generation in the driving control elements, there is a limitation in embedding multi-channels into the IC. Due to the distribution of the IC, an interchannel matching characteristic is degraded. There is a need a compensation circuit for solving those problems, which will increase the price of the device.